Melt butter over low heat in heavy 3-qt saucepan.
Add chopped onion and sauté until fragrant, approx. 2-3 min.
Add pork and blend well.
Fill saucepan with water to c. ½" above pork mixture.
Bring to boil over high heat. Reduce heat to medium-low. Cover and simmer two hours, stirring occasionally and breaking up any clumps.
Blend in poultry seasoning, allspice, nutmeg, salt, and pepper.
Cover and continue to simmer until water simmers away, leaving approx. ¼-½" rendered fat on top, approx. 1-2 hrs.
Pour into container and let cool approx. 20-30 min. Stir to blend rendered fat through. Cover and refrigerate until fully cold and solid, preferably overnight.
Garnish and serve as desired.